This piece uses miniature incandescent 7-segment displays.  Each digit is comprised of 7 glowing filaments encased in vacuum -- like small light bulbs only dimmer.

The circuit for each digit is directly below it, and built into the structure.  Each circuit counts from 0-9 repeatedly but at different speeds that are determined by the ambient light in the room.  There are tiny light sensors hidden in the structure that vary the speed.  In full sun the numbers all count very rapidly,  while in a dark room you will see each one change every 5 to 10 seconds.